For example, various loads mounted on a vehicle such as a head light, tail lamp, etc. are operated by application of the DC voltage supplied from a battery. As the case may be, an excess current flows through each load because of malfunction of a circuit, operation failure, etc. When the excess current flows, a semiconductor switch and a harness for connecting a power source circuit with the load are excessively heated so that This may lead to a trouble that the harness is burnt.
In order to avoid the occurrence of such a trouble, an excess current protection apparatus has been put into practice which detects the current flowing through the load or semiconductor switch and cuts off the semiconductor switch when the current has become an excess current.
However, the related current protection apparatus only detects the excess current and does not consider-the-temperature of the semiconductor switch. Specifically, when the temperature of the semiconductor switch is high and the ambient temperature is also high, even if the current is not so large, the semiconductor switch may be damaged. On the contrary, when the temperature of the semiconductor switch is low, even if the current is large, the circuit may be cut off although there is no fear of damage.